Need some advice on Polyurethane finish

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Forum topic by WhoMe posted 01-23-2012 01:25 AM 11185 views 0 times favorited 16 replies Add to Favorites Watch
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1441 posts in 2666 days

01-23-2012 01:25 AM

I am in the process of finishing a set of oak kitchen cabinets and got my first coat of poly on them. This was brushed on straight from the can. Even though I put the coat on light to medium (versus a heavy thick coat), I found later that I got some minor sags in a couple of spots on the doors. Even though I don’t think my client will see them (but not 100% sure), I can see them and I am not going to try to hide the imperfections. Which doesn’t matter as I know they are there and I want to fix it and make it look “Right” and I also don’t want to give my client shoddy work since you never know if I will get further work from them or a new job based on my work.
Since I will be putting one more coat on before I am complete (maybe 2 on a couple of high traffic areas), what is the best way to sand these minor imperfections down. Should I use straight sand paper (with sanding block) or wet sand them with a dash of mineral spirits? I plan on using 320 grit sand paper?
Oh, and BTW, the cabinet doors where the imperfections are will have had about 3+ days of drying time on them.

Also, should I use straight poly for the final coat or thin it a bit? And If I thin the poly, how does this affect the drying time?

Thanks for any advice.

-- I'm not clumsy.. It's just the floor hates me, the tables and chairs are bullies, the wall gets in the way AAANNNDDD table saws BITE my fingers!!!.. - Mike -

16 replies so far

View RandyM68's profile


693 posts in 1741 days

#1 posted 01-23-2012 01:57 AM

I would try the sanding block first. I don’t know what mineral spiriits will do. I would put at least two more coats on it. Thinning the finish will make it dry faster, but I don’t know if you’re supposed to thin poly.

-- I can explain it to you, but I can't understand it for you. I'm sorry,thanks.

View Clint Searl's profile

Clint Searl

1533 posts in 1784 days

#2 posted 01-23-2012 02:03 AM

You should have taken the doors off so they could have been done flat. Take them off now, wet sand the sags with 220, then recoat.

-- Clint Searl....Ya can no more do what ya don't know how than ya can git back from where ya ain't been

View Manitario's profile


2393 posts in 2306 days

#3 posted 01-23-2012 02:11 AM

Personally I hate brush on poly for the reasons you mentioned. “Thinned poly” or wipe on poly works well, I usually give 8h or so between coats.

-- Sometimes the creative process requires foul language. -- Charles Neil

View JAAune's profile


1617 posts in 1740 days

#4 posted 01-23-2012 02:14 AM

I always thin polyurethane if I intend to brush it on furniture as I’ve found it much easier to apply a thin, level coat. Usually adding solvent will decrease the curing time for oil-based finishes but the type of thinner used will make a difference too. Naphtha is compatible with polyurethane and will evaporate faster than mineral spirits.

I haven’t tried wet-sanding polyurethane yet but I have done plenty of dry sanding. It’s the hardest finish to sand properly in my experience since you’ll need to avoid sanding through individual coats of poly. It can be done though as I’ve gotten a lot of compliments on the finish on my traveling carving tool box. That was just two or three coats of poly sanded between coats and finished with 0000 steel wool buffing and paste wax.

I’d recommend using the block as well. Change out sandpaper often as it starts to clog.

-- See my work at and

View CharlieM1958's profile


16229 posts in 3641 days

#5 posted 01-23-2012 02:18 AM

I agree with Clint that doing the doors flat would be a good idea. The good news is that the poly is pretty forgiving. Sand the sags off with a sanding block, and the next coat should cover everything.

For the record, I would never use a brush on cabinets. Wiping on requires more coats, but it’s less work, goes a lot faster, and the drying time between coats is relatively quick.

-- Charlie M. "Woodworking - patience = firewood"

View ajosephg's profile


1878 posts in 2984 days

#6 posted 01-23-2012 02:37 AM

I would use 220 grit wet dry sandpaper with water (and a block). Much less messy than mineral spirits. Use a cloth lightly soaked with mineral spirits to remove the white sanding “dust”. Let dry 10 or 15 minutes and recoat.

I strongly agree that it is much easier with the doors flat.

I’ve good luck with rattle can poly for the final coat, but it gets expensive if you have a large area to coat.

-- Joe

View 404 - Not Found's profile

404 - Not Found

2544 posts in 2392 days

#7 posted 01-23-2012 02:45 AM

Before you go at it with the sandpaper, I’d lay a sharp chisel flat against the run and cut it off level. Then flat it and the rest of it down for the second coat with P180. If you try sanding the raised part out you could end up with a halo around it where it’s goes back to bare.

View richgreer's profile


4541 posts in 2498 days

#8 posted 01-23-2012 05:33 AM

My approach is quite different. I always use wipe on poly which is quite thin. Furthermore, I brush it on with a disposable foam brush and I wipe it off with a lint-free paper towel almost immediately. Hence, I am applying a very thin coat. I usually apply 5 coats. When applying poly like this I can apply a fresh coat every 3-4 hours. I never have a problem with sags.

I very lightly sand with 400 grit sand paper before the 4th and 5th coat.

WIth respect to WhoMe, I would probably sand down what he has done quite aggressively and apply a series of thin coats of wipe on poly in a manner consistent with how I do it.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

View RandyM68's profile


693 posts in 1741 days

#9 posted 01-23-2012 06:00 PM

Renners has the right idea. I put the first coat on my bench top when it was almost too dark to see. The next morning, I had a big puddle that stuck out like a sore thumb. I sanded it as best as I could and put on two more coats which just built up and looked even worse. I was about ready to sand the whole thing down again and start over. I tried the chisel idea this morning, and it worked great. It left only minor scratches that easily covered up. And I had a six inch diameter puddle where I dumped a whole brush load and forgot to spread it out.

Rich, I like your idea about using wipe on poly. I’ve been using laquer because you can recoat in two hours instead of waiting twelve. I like poly better except for the dry time.Your way you can still do three or four coats a day.

-- I can explain it to you, but I can't understand it for you. I'm sorry,thanks.

View Dave's profile


11394 posts in 2263 days

#10 posted 01-24-2012 04:38 AM

I would use a large eraser with some 400 wet or dry wrapped around it. Dip it into mineral spirits and a touch of BLO. You have the problem and are going to put on a few more coats so breaking the gloss won’t be a problem. Keep in mind while its wet it will fool the eye. Use your bare fingers and feel it closely. Let it dry and look at it in a raking light. You want an even scratch pattern. No glossy streaks in it.

-- Superdav "No matter where you go - there you are."

View Dark_Lightning's profile


2620 posts in 2532 days

#11 posted 01-24-2012 05:04 AM

The thing to think about is, eventually, your clients will discover the sags. If you found them already, the best way to sleep at night is to fix them now, unless it is really minor. Otherwise, you may get some negative advertising, downstream.

-- Random Orbital Nailer

View Grandpa's profile


3256 posts in 2098 days

#12 posted 01-24-2012 06:15 AM

My paint contractor friend has a cheap aluminum ladder. He has driven black utility screws through the side rails. He uses this to place cabinet doors on when he shoots the varnish on them. Mount the doors and then take them off and remove the hinges. Apply the finish then hang them again. That is the standard practice. Polyurethane can be thinned with mineral spirits and it will lay faster when brushed. It will dry faster too so it has to lay faster. Repair the runs or sags now. If you find you can’t then replace the doors with new ones. Your reputation will thank you later. Ditto on the stuff said above. I would use a rubber sanding block with 220 on it for those pesky sags.

View CharlesNeil's profile


1602 posts in 3294 days

#13 posted 01-24-2012 03:12 PM

the issue with water base over any oil is adhesion, if its a pure tung oil it usually doesnt dry well , when a finish is dry you should be able to light scuff sand it and get a whitish powder, not a gummy goo, the safest way to apply a water over an oil is a seal coat of dewaxed shellac, then your assured adhesion is afforded, a quick easy shellac is “seal coat” by Zinnzer, or you can mix your own, also the yellow spray cans are dewaxed, the yellow and orange qt cans are not, good luck

View WhoMe's profile


1441 posts in 2666 days

#14 posted 01-24-2012 05:58 PM

Thanks all for the great advice luckily the sags are not real bad and I can only see them in raking light. And there is no question I will be fixing them.
One more question, how much should I thin the poly for wiping on?

-- I'm not clumsy.. It's just the floor hates me, the tables and chairs are bullies, the wall gets in the way AAANNNDDD table saws BITE my fingers!!!.. - Mike -

View CharlieM1958's profile


16229 posts in 3641 days

#15 posted 01-24-2012 06:33 PM

Personal preferences will vary, but 50/50 is a good place to start.

-- Charlie M. "Woodworking - patience = firewood"

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